Method for producing a metal-ceramic substrate

ABSTRACT

The invention relates to a method for producing a metal-ceramic substrate including first and second metallizations and at least one ceramic layer incorporated between the first and second metallizations. Advantageously, first and second metal layers and the at least one ceramic layer are stacked superposed, and in such a way that the free edge sections, of the first and second metal layers respectively, project beyond the edges of the at least one ceramic layer and the first and second metal layers are deformed toward each other in the region of the projecting free edge sections and directly connected to each other in order to form a gas-tight, sealed metal container enclosing a container interior for receiving the at least one ceramic layer. Subsequently, the metal layers forming the metal container with the at least one ceramic layer received in the container interior are hot isostatically pressed together in a treatment chamber at a gas pressure between 500 and 2000 bar and at a process temperature between 300° C. and the melting temperature of the metal layers for producing a preferably flat connection of at least one of the metal layers and the at least one ceramic layer, and at least the projecting free edge sections, which are connected to each other, of the metal layers for forming the first and second metallization are subsequently removed.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing a metal-ceramicsubstrate including a first and second metallization and at least oneceramic layer accommodated between the first and second metallization,wherein first and second metal layers and the at least one ceramic layerare stacked superposed in such a way that free edge sections of thefirst and second metal layers respectively project beyond the edges ofthe at least one ceramic layer forming projecting free edge sections.

Metal-ceramic substrates in the form of printed circuit boardscomprising a ceramic layer and at least one metallization connected to asurface side of the ceramic layer and structured for the formation ofstrip conductors, contacts, contact faces or terminal faces are known inthe most diverse embodiments. Such metal-ceramic substrates are used forexample for constructing power semiconductor modules, i.e. are intendedfor higher operational voltages, namely 600 V and more. One of therequirements on such power semiconductor modules is a sufficiently highpartial discharge resistance, wherein metal-ceramic substrates also haveto meet this requirement.

Furthermore, the so-called “DCB process” (“Direct-Copper-Bonding”) isknown for connecting the metallization-forming metal foils or metallayers to one another or to a ceramic substrate or a ceramic layer.Metal layers, preferably copper layers or copper foils, are connected toone another and/or to a ceramic layer, namely using metal or coppersheets or metal or copper foils, which at their surface sides comprise alayer or coat (“fusing layer”) of a chemical compound of the metal and areactive gas, preferably oxygen. In the case of this process describedby way of example in US-PS 37 44 120 or in DE-PS 23 19 854, this layeror this coat (“fusing layer”) forms a eutectic with a meltingtemperature below the melting temperature of the metal (e.g. copper), sothat by placing the metal or copper foil on the ceramic layer and byheating all the layers, the latter can be connected together, namely bymelting of the metal layer or copper layer essentially only in theregion of the fusing layer or oxide layer. One such DCB processcomprises for example the following process steps:

-   -   oxidation of a copper foil in such a way that a uniform copper        oxide layer results;    -   placing the copper foil with the uniform copper oxide layer onto        the ceramic layer;    -   heating of the composite to a process temperature between 1025        to 1083° C., for example to approx. 1071° C.;    -   cooling to room temperature.

A drawback of the DCB process consists in the fact that process-relatedimperfections can arise between the respective copper layer and theceramic layer. Indeed, these imperfections scarcely impair the thermalproperties of a metal-ceramic substrate produced using the DCB process,but an impairment of the partial discharge resistance of the powersemiconductor module produced therefrom results.

Furthermore, the so-called active soldering method for connectingmetallization—forming metal layers or metal foils, in particular alsocopper layers or copper foils, to a ceramic material or a ceramic layeris known from documents DE 22 13 115 and EP-A-153 618. With this method,which is also used especially for the production of metal-ceramicsubstrates, a connection is produced between a metal foil, for examplecopper foil, and a ceramic substrate, for example an aluminium nitrideceramic, at a temperature between approx. 800-1000° C. using a hardsolder, which also contains an active metal in addition to a maincomponent, such as copper, silver and/or gold. This active metal, whichis for example at least one element of the group Hf, Ti, Zr, Nb, Ce,produces a connection between the hard solder and the ceramic bychemical reaction, whereas the connection between the hard solder andthe metal is a metallic hard solder joint. A drawback, however, is thatthe required hard solder is very cost-intensive and the structuring ofthe metallization applied by means of an active soldering method iscostly from the process standpoint.

A method of producing a stack comprising a plurality of metal plates forforming a cooler is already shown in EP 1 716 624 B1, wherein aplurality of thin metal plates or metal foils are connected to oneanother to form a stack, which then form a cooler, in particular amicro-cooler. To prevent the formation of micro-cavities in thetransition regions between the metal plates, a post-treatment of theplate stack takes place in a protective gas atmosphere at a hightreatment temperature below the jointing temperature and at a high gaspressure in the region between 200 and 2000 bar. This post-treatment isalso referred to as hot-isostatic pressing (“HIP process”). The exposureof the plate stack to high gas pressure in the protective gas atmosphereunder the stated temperature conditions leads, amongst other things, tothe fact that the connection between the plates is for the most partfree from micro-cavities, i.e. there are no recesses or holes in theconnection region of two metal plates. As a protective gas, use is madehere of nitrogen, argon or other inert or noble gases. The treatmenttemperature is adjusted such that diffusion bonding arises between theadjacent surfaces of the metal plates.

A method for producing a metal-ceramic substrate is known from EP 1 774841 B1, wherein a copper layer is applied to at least one surface sideof a ceramic layer using the DCB process. Here, the metal-ceramicsubstrate is subjected in a following process step to a gas pressure inthe range from 400-2000 bar and is post-treated at a post-treatmenttemperature in the range between 450 and 1060° C. To preventimperfections or micro-cavities in the region of the metal-ceramictransition following the DCB process, the substrate is subjected to thedescribed post-treatment in a closed pressure chamber in a protectivegas atmosphere, for example an argon atmosphere, by heating to atemperature of approx. 560° C. at a pressure of approx. 1100 bar. Thebonding of the copper metallizations to the ceramic layer is thusenhanced and the formation of imperfections markedly reduced.

The aforementioned methods for eliminating imperfections have thedrawback of a high process-related outlay, especially since, in thefirst place, a direct flat jointing connection between the metal layerand the ceramic layer has to be produced by means of a first connectiontechnology and the latter subsequently has to be subjected to apost-treatment in order to eliminate micro-cavities arising in thejointing process.

A method for producing a direct flat connection of a ceramic layer to ametal layer is also already known from U.S. Pat. No. 4,763,828, whereindiffusion bonding in an argon protective gas atmosphere is describedusing the HIP process.

A multilayer casing material comprising a ceramic layer and a metallicmaterial for nuclear reactors is known from WO 2012/048 071 A1. A tubeor a channel is formed from this multilayer casing material. Themetallic material forms the “inner” layer of the tube or the channel andbrings about hermetic sealing of the latter.

Furthermore, a method for producing an electrical resistance comprisinga metal foil connected to a ceramic substrate is known from U.S. Pat.No. 4,325,183, wherein a direct flat connection between the metal foiland the ceramic substrate is produced by means of the HIP process. Forthis purpose, the arrangement comprising the metal foil and the ceramicsubstrate is accommodated in a sealed envelope, wherein the envelopelies adjacent in a tight manner and has previously been evacuated.

SUMMARY OF THE INVENTION

Based on the aforementioned art, the subject of the invention is toprovide a method for producing a metal-ceramic substrate, which enablesa cavity-free or microcavity-free flat and direct connection between theceramic layer and the metal layer, said method being able to beimplemented by means of few process steps and in a manner that sparesresources.

The main aspect of the method according to the invention is to be seenin the fact that the first and second metal layer and the ceramic layerare stacked superposed, namely in such a way that the free edge sectionsof the first and second metal layer respectively project beyond theedges of the ceramic layer, that the first and second metal layers aredeformed towards each other in the region of the projecting free edgesections and directly connected to each other in order to form agas-tight, sealed metal container enclosing a container interior foraccommodating the ceramic layer, that the metal layers forming the metalcontainer with the ceramic layer accommodated in the container interiorare hot isostatically pressed together in a treatment chamber at a gaspressure between 500 and 2000 bar and at a process temperature between300° C. and the melting temperature of the metal layers to produce aconnection, wherein at least the projecting free edge sections of themetal layer which are connected to each other are removed for formingthe first and second metallizations. The metal layers overlapping at theedges are particularly preferably used for the gas-tight encapsulationof the ceramic layer for the subsequent HIP process. A particularlyefficient and resource-saving method for the direct flat connection of ametal layer to a ceramic layer thus results, and moreover free fromdisruptive cavities or micro-cavities. Furthermore, when use is made ofthe HIP process, as compared to the DCB process, thinner metal layerscan advantageously be processed and applied onto a ceramic layer. Theminimum thickness of the metal layers that can be processed by means ofthe DCB process amounts to approx. 200 micrometer, whereas metal layerswith a layer thickness of approx. 50 micrometer and over can be appliedto the ceramic layer when use is made of the HIP process. Furthermore,the grain sizes in the HIP process can also be advantageously adjusted,in contrast with the DCB process.

According to an advantageous embodiment of the method according to theinvention, the free edge sections of the metal layers are deformedbefore their direct connection by applying a clamping force to form themetal container and the free edge sections of the metal layers are thenjoined together at the edges by welding, in particular contact weldingor laser welding, or by soldering, also hard soldering. Alternatively,the direct connection of the metal layers at the edges can be producedby means of a mechanical-connection and/or processing method, and moreprecisely by rolling, pressing and/or flanging of the free edgesections.

The container interior is particularly advantageously evacuated beforethe direct connection of the metal layers at the edges, wherein oxygencan also be introduced into the evacuated container interior before thedirect connection of the metal layers at the edges. An optimumatmosphere for the HIP process is formed by the evacuation andintroduction of oxygen, in particular an oxide layer arises on theceramic layer and/or the metal layer, by means of which the adhesivenessof the diffusion bonding is further increased.

The metal layers are also advantageously connected directly to oneanother at the edges in a vacuum or in an inert gas atmosphere,preferably using nitrogen or argon as an inert gas.

According to an advantageous variant of embodiment, a porous material,in particular a porous auxiliary ceramic layer, is introduced into thecontainer interior in addition to the ceramic layer, said auxiliaryceramic layer being designed for absorbing a residual gas present in thecontainer interior. Inclusions produced by performing the HIP processdue to residual gas that is present can thus be prevented. The porousmaterial is preferably evacuated before introduction into the containerinterior and coated with a gas-tight sealing layer. The porous materialcan also be charged with oxygen before introduction in the containerinterior and coated with a gas-tight sealing layer.

Before the stacking, the ceramic layer can also be completely coated atits upper and/or lower side with a hard solder layer or an active solderlayer, as a result of which additional hard soldering of the metal layerand the ceramic layer is possible with a reduced quantity of solder.

Furthermore, the subject-matter of the present invention is analternative method for producing a metal-ceramic substrate comprising afirst and second metallization and at least one ceramic layer with anupper and lower side accommodated between the first and secondmetallization, wherein a peripheral, frame-like, preferably continuoussolder layer is deposited at the edges on the upper and lower side ofthe ceramic layer, wherein a first and second metal layer and theceramic layer are stacked superposed and a solder joint is produced atthe edges by heating at least in the region of the peripheral,frame-like solder layer, namely such that a gas-tight sealed, gap-likeintermediate space arises between the ceramic layer and the respectivemetal layer, wherein the metal layers and the ceramic layer are pressedtogether hot isostatically in a treatment chamber at a gas pressurebetween 500 and 2000 bar and a process temperature between 300° C. andthe melting temperature of the metal layers or of the solder layer andwherein at least the edge regions of the metal layers connected to theceramic layer by the peripheral, frame-like solder joint are removed forforming the first and second metallization.

Preferably, at least the edge regions of the metal layers connected tothe ceramic layer by means of the solder joint at the edges are removedfor forming the first and second metallization.

In a variant of embodiment of the two methods, one of the metal layers,before the hot isostatic pressing, is connected to the ceramic layerusing a direct copper bonding process or a hard soldering or activesoldering method. In particular, it may be desirable to form only one ofthe metallizations with a layer thickness from 50 micrometer, whereas ametallization with a layer thickness from 200 micrometer producedaccording to known connection methods is sufficient for the oppositefurther metallization, for example for connecting a cooling arrangement.

Furthermore, at least one further metal layer, preferably of copper oraluminium, can advantageously be introduced between the first metallayer and the ceramic layer and/or between the second metal layer andthe ceramic layer during the stacking. A first metal layer of copper anda further metal layer of aluminium are preferably provided, which areeither loosely stacked superposed or directly connected to one another,for example welded or roll-bonded to one another. The further metallayer can have different dimensions from the dimensions of the ceramiclayer, for example can be accommodated inside the edge region of theceramic layer or run flush with the latter or extend over the edgeregion of the ceramic layer, but at most to an extent such that thefirst and further ceramic layer have a flush edge profile.

Furthermore, the layers are advantageously clamped against one anotherby means of an auxiliary tool comprising two, preferably mutuallyopposite half-shells for the connection of the metal layers at theedges.

According to a preferred variant of embodiment of the method accordingto the invention, at least one of the metal layers and/or the upperand/or lower side of the ceramic layer is provided with an oxide layerbefore the stacking. For example, the surface of the metal layersprovided for the connection can be oxidised or the ceramic layer canundergo a preliminary treatment and thermal oxidation.

The expressions “approximately”, “essentially” or “roughly”, within themeaning of the invention, signify deviations from the exact value of+/−10%, preferably of +/−5% and/or deviations in the form of changeswhich are unimportant for the function.

Developments, advantages and possible applications of the invention alsoemerge from the following description of examples of embodiment and fromthe drawings. All the described and/or figuratively illustrated featuresin themselves or in any combination are in principle the subject-matterof the invention, irrespective of their combination in the claims ortheir back-reference. The content of the claims also forms a componentpart of the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below with the aid of thefigures using examples of embodiment. In the figures:

FIG. 1 shows a simplified schematic cross-sectional representationthrough a metal-ceramic substrate,

FIG. 2 shows a simplified cross-sectional representation through a stackcomprising first and second metal layers and a ceramic layeraccommodated in between,

FIG. 3 shows a schematic cross-sectional representation through a metalcontainer formed from the stack according to FIG. 2 with a ceramic layeraccommodated in the container interior,

FIG. 4 shows a schematic plan view of the metal container according toFIG. 3,

FIG. 5 shows a schematic cross-sectional representation through a metalcontainer with a ceramic layer accommodated in the container interiorand an additional porous auxiliary ceramic layer,

FIG. 6 shows a simplified schematic cross-sectional representationthrough the metal container according to FIG. 3 accommodated in anauxiliary tool,

FIG. 7 shows a simplified schematic cross-sectional representationthrough a stack comprising first and second metal layers and a ceramiclayer accommodated in between, which are connected at the edges by aperipheral, frame-like solder joint,

FIG. 8 shows a schematic plan view of the stack according to FIG. 6, and

FIGS. 9a-d show several variants of embodiment of a stack comprisingfour metal layers and a ceramic layer for producing a metal-ceramicsubstrate according to the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, in a simplified schematic representation, a cross-sectionthrough a standard embodiment of a metal-ceramic substrate 1 comprisinga ceramic layer 2 with an upper side 2.1 and a lower side 2.2, which areprovided respectively with a metallization 3, 4, namely the upper side2.1 comprises a first metallization 3 and the lower side 2.2 a secondmetallization 4.

Metallizations 3, 4 can be produced from the same or different metal(s)and can be constituted structured for the formation of contact surfacesand/or an electronic circuit. First and second metallizations 3, 4 arepreferably connected directly and flat with the upper and lower side2.1, 2.2, respectively.

First and second metallizations 3, 4 are produced for example fromcopper or a copper alloy or aluminium or an aluminium alloy. The layerthickness of the first and second metallizations 3, 4 amounts to atleast 20 micrometer, preferably between 20 and 900 micrometer, whereinlayer thicknesses between 150 and 600 micrometer preferably being usedin the area of microelectronics and layer thicknesses between 50 and 150micrometer preferably being used in the LED area. It goes without sayingthat other suitable metals can also be used to produce metallizations 3,4.

Ceramic layer 2 is produced for example from an oxide, nitride orcarbide ceramic such as aluminium oxide (Al₂O₃) or aluminium nitride(AlN) or silicon nitride (Si₃N₄) or silicon carbide (SiC) or fromaluminium oxide with zirconium oxide (Al₂O₃+ZrO₂) and has a layerthickness for example between 50 micrometer and 1000 micrometer,preferably between 200 micrometer and 700 micrometer.

This is where the invention comes in and proposes an efficient methodfor producing a metal-ceramic substrate 1 comprising such first andsecond metallizations 3, 4 and at least one ceramic layer 2 accommodatedbetween first and second metallizations 3, 4. According to theinvention, the direct flat connection between respective metallizations3, 4 and ceramic layer 2 is produced by using a pressure-inducedconnection process, and more precisely the hot-isostatic pressingprocess (HIP process), in order to effectively prevent the emergence ofcavities or micro-cavities in the connection region between respectivemetallizations 3, 4 and ceramic layer 2. Much thinner metal layers 5, 6can also be processed by means of the hot-isostatic pressing process ascompared to the DCB process, for example even from a minimum layerthickness of approx. 50 micrometer.

The basic mode of operation of the hot-isostatic pressing process or theso-called HIP method or HIP process is known. The pressure preferablyapplied in a two-dimensionally extending manner to the layers to beconnected can be generated mechanically and/or by means of a gas orfluid. The layer thickness can advantageously be reduced with the use ofthe HIP process for producing a preferably flat connection betweenmetallizations 3, 4 and ceramic layer 2 as compared to the otherconnection methods, in particular metal layers having a thickness of 50micrometer and over can be processed.

For the pressure-induced flat connection of first and secondmetallizations 3, 4 to ceramic layer 2, first and second metal layers 5,6 are the first provided, which are stacked superposed together withceramic layer 2, and more precisely such that ceramic layer 2 isaccommodated between metal layers 5, 6. The layer sequence ofmetal-ceramic substrate 1 represented in FIG. 1 is thus already present,wherein first and second metal layers 5, 6 and ceramic layer 2 do notyet have a flat connection.

First and second metal layers 5, 6 are adapted in terms of theircross-sectional shape essentially to the cross-sectional shape ofceramic layer 2. Ceramic layer 2 preferably has a rectangular or squarecross-sectional shape. Alternatively, however, ceramic layer 2 can alsohave a round, oval or an otherwise polygonal cross-sectional shape.

First and second metal layers 5, 6 overlap with their free edge sections5 a, 6 a free edge 2′ of ceramic layer 2, namely over the entire courseof free edge 2′ of ceramic layer 2. First and second metal layers 5, 6thus project outwards with their free edge sections 5 a, 6 a from freeedge 2 a of ceramic layer 2, and for example over a length L of between3 mm and 30 mm Length L is selected here depending on the layerthickness of first and second metal layers 5, 6.

In the present example of embodiment, a stack comprising first andsecond metal layers 5, 6 and ceramic layer 2 is formed according to FIG.2, wherein ceramic layer 2 lies flat on second metal layer 6 and firstmetal layer 5 then follows on the latter, i.e. ceramic layer 2 isaccommodated between first and second metal layers 5, 6 in the stack andfree edge sections 5 a, 6 a of the first and second metal layers 5, 6project beyond ceramic layer 2 at the edges.

According to the invention, a gas-tight sealed metal container 7 with acontainer interior 8 is formed from first and second metal layers 5, 6,in which container interior ceramic layer 2 is completely accommodated.In this regard, first and second metal layers 5, 6 are deformed towardsone another in the region of projecting free edge sections 5 a, 6 a, andmore precisely by applying lateral clamping forces F, F′ preferablyacting perpendicular to the surface of metal layers 5, 6, which clampingforces are orientated respectively in the direction of upper and lowersides 2.1, 2.2 of ceramic layer 2. The flat sections of first and secondmetal layers 5, 6 directly adjacent to ceramic layer 2 preferably runparallel to upper side and lower sides 2.1, 2.2 of ceramic layer 2.Metal container 7 thus essentially comprises two half-shell shapedhalves 7.1, 7.2, which are formed by first and second deformed metallayers 5, 6.

In the present example of embodiment, free edge sections 5 a of firstmetal layer 5 are acted upon by a clamping force F acting verticallydownwards i.e. in the direction of ceramic layer 2, and free edgesections 6 a of second metal layer 6 are acted upon by a clamping forceF′ acting vertically upwards, i.e. in the direction of ceramic layer 2,and are thus clamped together and a corresponding deformation of metallayers 5, 6 in their free edge sections 5 a, 6 a is thus achieved.Correspondingly deformed free edge sections 5 a, 6 a are connecteddirectly together, and more precisely along a peripheral, i.e. annular,connection region VB. Free edge sections 5 a, 6 a preferably produce agas-tight sealed edge of metal container 7 running flush. Metalcontainer 7 thus forms an encapsulation of ceramic layer 2 by means ofcontainer halves 7.1, 7.2 constituted half-shell shaped and connectedtogether gas-tight.

The direct connection of deformed free edge sections 5 a, 6 a at theedges in annular connection region VB preferably takes place by welding,in particular by contact welding or laser welding or by soldering, alsohard soldering. Alternatively, the direct connection at the edges canalso be produced by means of a mechanical-connection and/or processingmethod, and more precisely by rolling, pressing and/or flanging of freeedge sections 5 a, 6 a, wherein a gas-tight mechanical connection at theedges between the two metal layers 5, 6 also arises here along annularconnection region VB. With a connection by means of laser welding, freeedge sections 5 a, 6 a are preferably welded to one another at the outeredges, and preferably at an angle between 45° and 90°. In a variant ofembodiment, the direct connection at the edges takes place in anatmosphere advantageous for the connection process, for example in avacuum, in an air atmosphere or in an inert gas atmosphere using, forexample, nitrogen or argon as an inert gas.

FIG. 3 shows by way of example a schematic cross-section through metalcontainer 7 with ceramic layer 2 accommodated in container interior 8,from which the half-shell shaped embodiment of the container halves canalso be seen. FIG. 4 shows by way of example a plan view of metalcontainer 7 and the course of annular connection region VB indicated bya dashed line.

Metal layers 5, 6 forming metal container 1 with ceramic layer 2accommodated in container interior 8 are then pressed togetherhot-isotatically in a treatment chamber (not represented in the figures)at a gas pressure between 500 and 2000 bar and a process temperaturebetween 300° C. and the melting temperature of metal layers 5, 6. Adiffusion bond is thus produced between upper side 2.1 of ceramic layer2 and the section of the first metal layer 5 in direct connection withthe latter and between lower side 2.2 of ceramic layer 2 and the sectionof second metal layer 6 in direct connection with the latter, saiddiffusion bond comprising no cavities or micro-cavities and having ahigh adhesive strength.

Metal container 1 is subjected to a clamping force F, F′ and thusclamped in the region of free edge sections 5 a, 6 a, preferably alongannular connection region VB, by means of an auxiliary tool 9 with forexample two half shells 9.1, 9.2. This simplifies the production of adirect connection at the edges between free edge sections 5 a, 6 a, inparticular of a laser welding connection. A working medium such asoxygen or suchlike can thus also be introduced into container interior8, without the latter already being permanently sealed. FIG. 6 shows byway of example a cross-section through metal container 1 accommodated inauxiliary tool 9.

After the hot-isostatic pressing, at least in sections, of metal layers5, 6 or container halves 7.1, 7.2 with ceramic layer 2, free edgesections 5 a, 6 a are removed, preferably by a mechanical processingmethod or a suitable etching process. The remaining sections of firstand second metal layers 5, 6 connected to upper and lower sides 2.1, 2.2of ceramic layer 2 form first and second metallizations 3, 4 ofmetal-ceramic substrate 1.

In a variant of embodiment, the subjecting of metal layers 5, 6 toclamping force F, F′ and the subsequent connection at the edges of freeedge sections 5 a, 6 a takes place in the ambient atmosphere, i.e. evenafter the sealing of metal container 7 forming an encapsulation ofceramic layer 2, there is still at least residual air or a residual gaspresent in container interior 8. Due to the residual air or the residualgas, however, inclusions can arise in the connection region betweenmetal layers 5, 6 and ceramic layer 2 when the HIP process is carriedout.

In a variant of embodiment, metal layers 5, 6 and ceramic layer 2 arefirst stacked and clamped under an air atmosphere, for example by meansof auxiliary tool 9, and are then directly connected to one another atthe edges.

According to a further variant of embodiment, the container interior 8is evacuated and therefore the residual air present in containerinterior 8 or the residual gas present therein is preferably completelyremoved before the direct connection of free edge sections 5 a, 6 a ofmetal layers 5, 6 at the edges in order to prevent inclusions.

Furthermore, the adhesive strength of the pressure-induced directconnection can be increased by producing an oxide layer on upper orlower sides 2.1, 2.2 during the HIP process. The oxygen proportion inthe ambient atmosphere, however, is often too small for a suitable oxidelayer to form on metal layers 5, 6 and/or ceramic layer 2. Such an oxidelayer preferably comprises copper doping.

In order to assist the formation of a suitable oxide layer, oxygen canbe fed to the production process, and more precisely metal layers 5, 6can already be exposed to oxygen during the forming of the stack.

Alternatively or in addition, oxygen can be introduced into containerinterior 8 before the connection of metal layers 5, 6 at the edges andmetal layers 5, 6 can be pretreated with oxygen. Such flooding ofcontainer interior 8 with oxygen takes place for example before the edgeclamping of metal layers 5, 6 or after the evacuation of containerinterior 8.

The formation of an oxide layer can also be effectively assisted by apreliminary treatment of ceramic layer 2, and for example by applying anauxiliary layer by means of a mechanical-chemical process. In thismechanical-chemical process, a layer of copper, copper oxide or othercopper-containing compounds is deposited on at least one side 2.1, 2.2of ceramic layer 2, in particular an AIN substrate to produce theauxiliary layer. The deposition of this auxiliary layer can be carriedout using alternative methods. Copper, copper oxide or othercopper-containing compounds can be deposited for example by means ofsputtering processes, currentless deposition of copper with a standardbath, vapour deposition, screen-printing, immersion in solutions etc.Ceramic layer 2, in particular the AIN substrate, then undergoes anoxidation process by means of which copper, copper oxide or other coppercompounds are oxidised.

In particular, when use is made of an aluminium nitride ceramic layer 2for producing metal-ceramic substrate 1, the mechanical interlocking ofmutually opposite layers 2, 5, 6 arising in the performance of the HIPpressing process already has sufficient adhesive strength, i.e.additional production of an intermediate layer is not necessary, sincean oxide layer that is sufficient to create a connection forms by itselfon the surface of aluminium nitride ceramic layer 2 on account of theambient oxygen.

When use is made of a silicon nitride ceramic layer 2, a preliminarytreatment to produce an intermediate layer is likewise not necessary.The ambient oxygen or the residual oxygen in the container interior issufficient to allow the formation of a silicon dioxide layeradvantageous for the connection process.

In order to promote the formation of a natural oxide layer on theceramic surface, metal layers 5, 6 to be connected to ceramic layer 2can be enriched with oxygen. This oxygen can be released by the enrichedmetal layers 5, 6 during the HIP pressing process.

In a variant of embodiment, rinsing of container interior 8 after theclamping is conceivable with an inert gas, for example oxygen or argon.

In an alternative variant of embodiment, a porous material 11, forexample metal or ceramic, can be accommodated in container interior 8 inaddition to ceramic layer 2 in order to absorb the residual air or theresidual gas present in container interior 8, said porous materialforming an absorption reservoir for the residual gas present incontainer interior 8 during the performance of the HIP process. Anatmosphere advantageous for performing the HIP process is created bymeans of porous material 11.

Porous material 11 is preferably evacuated before introduction intocontainer interior 8. In a variant of embodiment, porous material 11 isprovided with a gas-tight sealing layer 12 before introduction intocontainer interior 8, porous materials 11 without a gas-tight sealinglayer 12 also being able to be used. In the performance of the HIPprocess, gas-tight sealing layer 12 is split open and the residual gaspresent in container interior 8 is absorbed in a targeted manner byevacuated porous auxiliary ceramic layer 11.

Alternatively or in addition, porous material 11, in particular prior tothe application of a gas-tight sealing layer 12, can be charged withoxygen in an oxygen atmosphere. Gas-tight sealing layer 12 in each caseforms a gas-tight casing of porous material 11. FIG. 5 shows by way ofexample a cross-section through a metal container 7 according to FIG. 3,in which at least one porous auxiliary ceramic layer 11 with a gas-tightsealing layer 12 is accommodated in addition to ceramic layer 2. It goeswithout saying that a plurality of layers of such porous materials 11can also be provided in a different form and arrangement inside metalcontainer 7.

Porous material 11 can also be provided for example with a copper oxidelayer. Such a CuO layer is converted in the HIP process into oxygen andCu₂O. The oxygen hereby liberated contributes to the oxidation of metallayers 5, 6 and of ceramic layer 2. Apart from CuO, use can also be madeof other oxides which, under the process parameters of the HIP process,in particular temperature and pressure, liberate oxygen and promote theformation of an oxide layer, and more precisely for example MnO, VO, TiOand Mo₃O.

In a further variant of embodiment, first metal layer 5 is produced fromcopper or a copper alloy and second metal layer 6 is produced fromaluminium or an aluminium alloy.

Also, for example, a further metal layer 5′ of aluminium can be providedbetween ceramic layer 2 and a first metal layer 5 of copper, which arealso stacked superposed. Analogous hereto, a further metal layer 6′ canalso be accommodated between second metal layer 6 and ceramic layer 2.First and second metal layers 5, 6 and respectively further metal layers5′, 6′ are preferably connected together by means of roll bonding.

FIG. 9 shows by way of example different variants of embodiment (a) to(d) of a metal-ceramic substrate 1 with a first metal layer 5 of copperand a further metal layer 5′ of aluminium and as well as a second metallayer 6 of copper and a further metal layer 6′ of aluminium, which areconnected together by means of the method according to the invention. Invariant of embodiment (c), metal layers 5, 6, 5′, 6′ are loosely stackedsuperposed, whereas in variant of embodiment (d) a connection existsbetween the upper and lower two metal layers 5, 5′ and respectively 6,6′. The connection can be produced either by roll bonding of the copperlayers and aluminium layers 5, 5′ and 6, 6′ or by welding at the edges.In variant of embodiment (a), further metal layers 5′, 6′ extend onlypartially over upper and lower sides 2.1, 2.2 of the ceramic layer, i.e.it is arranged offset inwards at the edge. Variant of embodiment (b)shows a flush edge course of further metal layers 5′, 6′ with ceramiclayer 2 and according to variant of embodiment (c) further metal layers5′, 6′ extend over free edge 2′ of ceramic layer 2, but inside the edgeof first and second metal layers 5, 6. In variant of embodiment (d),first metal layer 5 and assigned further metal layer 5′ and/or secondmetal layer 6 and further metal layer 6′ each have a flush edge course,i.e. they are constituted identical to one another with regard to shape.Furthermore, it is possible to provide a different layer structure onupper and lower sides 2.1, 2.2 of ceramic layer 2 or to provide acombination of variants of embodiment (a) to (d) represented in FIG. 9.

Furthermore, ceramic layer 2 or its upper and/or lower sides 2.1, 2.2can be coated over its entire area with a hard solder layer before themethod according to the invention is carried out. The soldering processis, as it were, thus integrated into the HIP process. The layerthickness of the hard solder layer can be advantageously reduced herecompared to the known active soldering method, i.e. a smaller quantityof solder is advantageously required, because as a result of the HIPprocess metal layers 5, 6 are also pressed into unevennesses of upperand/or lower sides 2.1, 2.2.

FIGS. 7 to 8 describe a method for producing a metal-ceramic substrate1, this method being an alternative method for encapsulating ceramiclayer 2 in a metal container 7. According to this method, ceramic layer2 is provided at the edges with a peripheral, frame-like solder layer 13at upper and lower sides 2.1, 2.2 and a stack comprising first andsecond metal layers 5, 6 and ceramic layer 2 is then formed.

In a subsequent step, the stack is heated in such a way that theprepared peripheral, frame-like solder layer 13 leads to a solder joint14 at the edges between respective metal layers 5, 6 and ceramic layer2. As a result of the heating and the previous evacuation of gap-likeintermediate spaces 15, 15′ between respective metal layers 5, 6 andceramic layer 2, an advantageous atmosphere is present for performingthe hot-isostatic process or HIP process.

Solder joint 14 at the edges also forms a preferably annular connectionregion VB′. Finally, the stack comprising first and second metal layers5, 6 and ceramic layer 2 and formed by solder joint 14 at the edges isin turn introduced into a treatment chamber (not represented) and, at agas pressure between 500 and 2000 bar and a process temperature between300° C. and the melting temperature of metal layers 5, 6 or of solderlayer 13, said metal layers are hot-isostatically pressed with ceramiclayer 2. The sections of respective metal layers 5, 6 connected in atwo-dimensionally extending manner to ceramic layer 2 formmetallizations 3, 4 of metal-ceramic substrate 1.

Edge sections 5 b, 6 b of metal layers 5, 6 connected by means ofceramic layer 2 via solder joint 14 at the edges are removed to delimitfirst and second metallizations 3, 4, and more precisely by means of amechanical processing method or by means of a laser or a suitableetching process. The remaining sections of first and second metal layers5, 6 thus form first and second metallizations 3, 4 of metal-ceramicsubstrate 1.

Peripheral, frame-like solder layer 13 is for example constitutedcontinuous, and preferably in the form of a closed ring. FIG. 8 shows byway of example the course of peripheral, frame-like solder layer 13deposited on upper side 2.1 of ceramic layer 2.

In a variant of embodiment of the two methods, one of the metal layersis connected to the ceramic layer, prior to the hot-isostatic pressing,by using a direct copper bonding process or a hot soldering or activesoldering method. Metallizations with a different layer thickness from50 micrometer can thus be produced.

The invention has been described above on the basis of examples ofembodiment. It goes without saying that numerous changes andmodifications are possible without thereby departing from the inventiveidea underlying the invention.

LIST OF REFERENCE NUMBERS

-   1 metal-ceramic substrate-   2 ceramic layer-   2′ free edge-   2.1 upper side-   2.2 lower side-   3 first metallization-   4 second metallization-   5 first metal layer-   5′ further metal layer-   5 a free edge section-   5 b edge region-   6 second metal layer-   6′ further metal layer-   6 a free edge section-   6 b edge region-   7 metal container-   7.1, 7.2 container halves-   8 container interior-   9 auxiliary tool-   9.1, 9.2 half-shells-   11 porous material-   12 gas-tight sealing layer-   13 peripheral, frame-like solder layer-   14 soldering joint at edges-   15, 15′ gap-like intermediate space-   VB, VB′ annular connection region-   F, F′ clamping forces

1-19. (canceled)
 20. A method for producing a metal-ceramic substratecomprising first and second metallizations and at least one ceramiclayer accommodated between the first and second metallizations, whereinfirst and second metal layers and the at least one ceramic layer arestacked superposed in such a way that free edge sections of the firstand second metal layers respectively project beyond the edges of the atleast one ceramic layer forming projecting fee edge sections, comprisingthe steps of: deforming the first and second metal layers towards eachother in a region of the projecting free edge sections and theprojecting free edge sections are directly connected to each other inorder to form a gas-tight, sealed metal container enclosing a containerinterior for accommodating the at least one ceramic layer, the first andsecond metal layers forming the gas-tight, sealed metal container withthe at least one ceramic layer accommodated in the container interior;and hot isostatically pressing together the first and second metallayers in a treatment chamber at a gas pressure between 500 and 2000 barand at a process temperature between 300° C. and a melting temperatureof the first and second metal layers in order to produce a flatconnection of the first and second metal layers and the at least oneceramic layer, wherein at least the projecting free edge sections of thefirst and second metal layers which are connected to each other areremoved for forming the first and second metallizations.
 21. The methodaccording to claim 20, wherein for a formation of the metal container,the projecting free edge sections of the first and second metal layersare deformed by an action of a clamping force before their directconnection.
 22. The method according to claim 20, wherein the projectingfree edge sections of the first and second metal layers are connectedtogether at edges of the first and second metal layers by welding,contact welding, laser welding, soldering, or by hard soldering, or thedirect connection of the first and second metal layers at the edges ofthe first and second metal layers is produced by amechanical-connection, by rolling, pressing and/or flanging of the freeedge sections.
 23. The method according to claim 20, wherein thecontainer interior is evacuated before the direct connection of theedges of the first and second metal layers.
 24. The method according toclaim 23, wherein oxygen is introduced into the evacuated containerinterior before the direct connection of the edges of the first andsecond metal layers.
 25. The method according to claim 23, wherein thefirst and second metal layers are connected directly to one another atthe edges of the first and second metal layers in a vacuum or in aninert gas atmosphere, using nitrogen or argon as the inert gas.
 26. Themethod according to claim 20, wherein a porous material is introducedinto the container interior in addition to the at least one ceramiclayer, the porous material absorbs a residual gas present in thecontainer interior.
 27. The method according to claim 26, wherein theporous material is evacuated and coated with a gas-tight sealing layerbefore being introduced into the container interior.
 28. The methodaccording to claim 26, wherein the porous material is charged withoxygen and coated with a gas-tight sealing layer before being introducedinto the container interior.
 29. The method according claim 20, whereinthe at least one ceramic layer is completely coated at its upper and/orlower side with a hard solder layer or an active solder layer beforestacking.
 30. The method according to claim 20, wherein the first and/orsecond metallization are produced from copper or a copper alloy and/oran aluminium or an aluminium alloy.
 31. The method according to claim20, wherein the at least one ceramic layer is produced from an oxide, anitride or carbide ceramic, aluminium oxide (Al₂O₃), an aluminiumnitride (AlN), a silicon nitride (Si₃N₄), a silicon carbide (SiC) or analuminium oxide with zirconium oxide (Al₂O₃+ZrO₂).
 32. A method forproducing a metal-ceramic substrate comprising first and secondmetallizations and at least one ceramic layer with upper and lower sidesaccommodated between the first and second metallizations, comprising thesteps of: depositing a peripheral, frame-like continuous solder layer atedges on the upper and/or lower side of the at least one ceramic layer;stacking first and second metal layers and the at least one ceramiclayer superposed, and producing a solder joint at the edges by heatingat least in a region of the peripheral, frame-like continuous solderlayer in such a way that a gas-tight sealed, gap-like intermediate spacearises between the at least one ceramic layer and the first and secondmetal layers; and pressing together hot isostatically the first andsecond metal layers and the at least one ceramic layer in a treatmentchamber at a gas pressure between 500 and 2000 bar and a processtemperature between 300° C. and a melting temperature of the first andsecond metal layers of the peripheral, frame-like continuous solderlayer in order to produce a flat connection of the first and secondmetal layers and the at least one ceramic layer, wherein the first andsecond metal layers form the first and second metallizations.
 33. Themethod according to claim 32, wherein edge regions of the first andsecond metal layers connected to the at least one ceramic layer by thesolder joint at the edges are removed for forming the first and secondmetallizations.
 34. The method according to claim 32, wherein at leastone of the first and second metal layers is connected to the at leastone ceramic layer, prior to the hot-isostatic pressing using a directcopper bonding process or a hot soldering or active soldering method.35. The method according to claim 32, wherein at least one additionalmetal layer is introduced between the first metal layer and the at leastone ceramic layer and/or between the second metal layer and the at leastone ceramic layer during the stacking.
 36. The method according to claim32, wherein the first and second metal layers and the at least oneceramic layer are clamped against one another by an auxiliary toolcomprising two half-shells for the connection of edges of the first andsecond metal layers.
 37. The method according to claim 32, wherein atleast one of the first and second metal layers and an upper side and/ora lower side of the at least one ceramic layer are provided with anoxide layer before the stacking.
 38. A metal-ceramic substrate producedaccording to a method according to claim 20.